1. Field of the Invention
This invention generally relates to wireless communication antennas and, more particularly, to an effectively balanced dipole, formed from an unbalanced microstrip antenna, and suitable for use in a wireless communications device telephone.
2. Description of the Related Art
The size of portable wireless communications devices, such as telephones, continues to shrink, even as more functionality is added. As a result, the designers must increase the performance of components or device subsystems while reducing their size, or placing these components in less desirable locations. One such critical component is the wireless communications antenna. This antenna may be connected to a telephone transceiver, for example, or a global positioning system (GPS) receiver.
Wireless communications devices, a wireless telephone or laptop computer with a wireless transponder for example, are known to use simple cylindrical coil antennas as either the primary or secondary communication antennas. The resonance frequency of the antenna is responsive to its electrical length, which forms a portion of the operating frequency wavelength. The electrical length of a wireless device helical antenna is often an odd multiple of a quarter-wavelength, such as 3λ/4, 5λ/4, or λ/4, where λ is the wavelength of the operating frequency, and the effective wavelength is responsive to the dielectric constant of the proximate dielectric.
Wireless telephones can operate in a number of different frequency bands. In the US, the cellular band (AMPS), at around 850 megahertz (MHz), and the PCS (Personal Communication System) band, at around 1900 MHz, are used. Other frequency bands include the PCN (Personal Communication Network) at approximately 1800 MHz,
the GSM system (Groupe Speciale Mobile) at approximately 900 MHz, and the JDC (Japanese Digital Cellular) at approximately 800 and 1500 MHz. Other bands of interest are global positioning satellite (GPS) signals at approximately 1575 MHz and Bluetooth at approximately 2400 MHz.
Typically, better communication results are achieved using a whip antenna, as opposed to the above-mentioned helical antennas. Using a wireless telephone as an example, it is typical to use a combination of a helical and a whip antenna. In the standby mode with the whip antenna withdrawn, the wireless device uses the stubby, lower gain helical coil to maintain control channel communications. When a traffic channel is initiated (the phone rings), the user has the option of extending the higher gain whip antenna. Some devices combine the helical and whip antennas. Other devices disconnect the helical antenna when the whip antenna is extended. However, the whip antenna increases the overall form factor of the wireless telephone.
It is known to use a portion of a circuitboard, such as a dc power bus, as an electromagnetic radiator. This solution eliminates the problem of an antenna extending from the chassis body. However, these radiators are extremely inefficient “antennas”, typically providing poor gain and directionality. These types of radiators are also susceptible to crosstalk from other signals on the board. Further, these types of radiators can also propagate signals that interfere with digital or radio frequency (RF) on the circuitboard. Electromagnetic communications through these radiators can also be shielded by other circuits, circuit groundplanes, the chassis, or other circuitboards in the chassis.
Regardless of whether the antenna is formed as a helical coil, a whip, or a microstrip (printed circuitboard) antenna, a conventional dipole is fabricated in a balanced configuration. That is, the radiator and counterpoise are 180 degrees out of phase. The balanced transmission line provides the optimal interface for a balanced dipole antenna. However, the typical radio frequency (RF) electrical circuit, including wireless telephones, use unbalanced transmission lines. When an unbalanced transmission line is interfaced with a balanced antenna, a mismatch occurs, as the antenna counterpoise processes a different RF voltage potential than the transmission line ground. As a result, the transmission line ground radiates. Alternately stated, the transmission line ground becomes part of the antenna. This unintentional radiation degrades the intended electromagnetic radiation pattern, and may radiate into other sensitive electrical circuits.
Likewise, when an unbalanced dipole antenna is interfaced with a transmission line, a mismatch occurs. Without an antenna counterpoise, the transmission line ground radiates. Alternately stated, the transmission line ground becomes part of the antenna. This unintentional radiation degrades the intended electromagnetic radiation pattern, and may radiate into other sensitive electrical circuits.
FIG. 10 is a schematic diagram of a balun used for interfacing an unbalanced transmission line to a balanced antenna (prior art). A balanced-to-unbalanced balun is used to minimize RF current in the transmission line ground. The balun induces a current choke at a low impedance point. Alternately stated, a current is induced in the balun that is equal and opposite is phase to the current in the ground. Shown is a so-called bazooka balun that uses λ/4 decoupling stubs.
Baluns, such as the balun shown in FIG. 10, are typically used with coaxial cable or coax hardlines. Alternately for lower frequency applications, toroidal baluns made with wire can be formed around loaded or unloaded core materials. However, these types of baluns are not practical for use with microstrip transmission lines. Conventionally, when microstrip transmissions lines are interfaced with a balanced antenna, for example in applications where space is critical, the overall electromagnetic performance suffers due to the lack of a balanced-to-unbalanced balun. These same problems also exist with the use of coplanar and stripline transmission lines. Likewise, when a microstrip antenna is fabricated without a counterpoise, when space is a pressing concern for example, and interfaced to a microstrip transmission line, a mismatch will occur.
It would be advantageous if a practical balun could be developed for use in interfacing an unbalanced microstrip, coplanar, or stripline transmission line to an unbalanced microstrip antenna.